In the use of radiotherapy to treat cancer and other ailments, a powerful beam of the appropriate radiation is directed at the area of the patient that is affected. This beam is apt to kill living cells in its path, hence its use against cancerous cells, and therefore it is highly desirable to ensure that the beam is correctly aimed. Failure to do so may result in the unnecessary destruction of healthy cells of the patient.
Several methods are used to check this, and devices such as the Elekta™ Synergy™ device employ two sources of radiation, a high energy accelerator capable of creating a therapeutic beam and a lower energy X-ray tube for producing a diagnostic beam. Both are mounted on the same rotatable gantry, separated by 90°. Each has an associated flat-panel detector, for portal images and diagnostic images respectively.
In our earlier application WO-A-99/40759, we described a novel coupling cell for a linear accelerator that allowed the energy of the beam produced to be varied more easily than had hitherto been possible. In our subsequent application WO-A-01/11928 we described how that structure could be used to produce very low energy beams, suitable for diagnostic use, in an accelerator that was also able to produce high-energy therapeutic beams.
In both these earlier applications, the energy of the beam was adapted by rotating a vane in the cells coupling adjacent accelerating cells of the linear array. In our yet further application, WO-A-2006/097697, we described an adaptation of that structure which allowed the apparatus to produce closely interspersed pulses of high-energy and low-energy radiation beams. By rotating the vane at high speed and pulsing the radiation beam in a like manner, the energy of radiation pulses can be easily adapted by varying the frequency of the vane rotation.
The disclosure of each of these three prior applications is hereby incorporated by reference. The reader should note that this application develops the principles set out in those applications, which should therefore be read in conjunction with this application and whose disclosure should be taken to form part of the disclosure of this application.
This latter arrangement presents a problem in that the vane is internal to the linear accelerator and therefore under vacuum. The vane is rotationally driven by means of a “wobblestick” coupling. In this arrangement (see FIG. 1), a first shaft 50 extends from the rotatable vane inside the coupling cavity to the exterior of the linear accelerator. Outside the cavity, the shaft is bent at an angle and the angled end welded to a set of flexible bellows 52. A vacuum exists inside the bellows and around the first shaft. A second shaft 54 is bent at a corresponding angle, and couples loosely to the outside of the bellows 52 and hence the bent portion of the first shaft 50. In use, the second shaft 54 is driven rotationally as shown, with the rotational motion being conveyed to the first shaft 50 inside the coupling cavity, and causing the vane 22 to rotate.
However, the loose coupling between the drive shaft 54 and the internal shaft 50 places an upper limit on the speed with which the vane 22 can be driven. In addition, the flexible bellows 52 limit the temperature at which the linear accelerator can be “baked out”, effectively limiting the vacuum quality of the system.